The present invention relates to a liquid crystal microlens used as a means for forming an image in a lens array.
There is commonly known a contact-type sensor having a construction such as that of FIGS. 14 and 15.
Referring to FIG. 14, a sensor 110 has a frame 108 in which are mounted a linear light-emitting element (LED) array 105, a rod lens array 106, and light-receiving element array 104. The light-receiving element array 104 comprises a substrate 103 formed at the bottom of the frame 108, a protection film 102 mounted on the substrate 103, and a sensor IC 101 comprising a plurality of photoelectric converters. A transparent plate 107 on which a text sheet 109 is set is mounted on the upper portion of the frame 108.
In operation, a light beam from the LED array 105 irradiates the text sheet 109. The light beams diffused and reflected at a particular reading line of the sheet 109 passes through the rod lens array 106 so as to form an image on the text upon the sensor IC of the light-receiving element array 104. Information regarding the shades of the text sheet conveyed by the reflected light, taking the form of the intensity of light, is converted into an electric signal by the sensor IC 101 and serially outputted in accordance with the scanning direction. After scanning one line in the scanning direction, the next line in the direction perpendicular to the scanning direction is scanned. By repeating the scanning operation, two-dimensional information on the text sheet 109 is converted into an electric signal in time sequence. FIG. 15 shows the arrangement of the rod lens array 106 of the contact-type sensor 110 shown in FIG. 14 and the operation thereof.
The principle and the construction of the rod lens array 106 are described hereinafter with reference to FIGS. 16a to 16c. Each rod lens of the rod lens array 106 is a graded index lens, each having a refractive index distribution shown in FIG. 16a. FIG. 16b shows the transmission of a light beam through the rod lens.
In FIG. 16a, the distribution of the refractive index n can be approximately expressed as
n=n0(1xe2x88x92(A/2)r2) 
where n0 is the refractive index on the optical axis, r is the distance from the optical axis in a radial direction, and A is the constant of the refractive index. The light beams tend to travel slower in a range where the refractive index is large and faster where the refractive index is small.
Due to such a characteristic, the light beam entered in the rod lens follows a path according to the winding interval P, which depends on the distribution of the refractive index, and is emitted out from the opposite end of the lens as shown in FIGS. 16a and 16b. 
As shown in FIG. 16c, by setting an appropriate rod lens length Z0 in relation to the winding interval, an erecting image Qxe2x80x3 of an image Q equal in size thereto can be formed at the opposite side of the rod lens at a distance TC. The image forming operation is also described in FIG. 15.
The reference L0 in FIG. 16c is a working distance between the rod lens and the object Q (Qxe2x80x3).
The rod lens is provided with the following characteristics.
(1) The rod lens has end faces which are flat, and is light in weight.
(2) The condition of the formed image can be arbitrarily changed dependent on the length of the rod lens.
(3) The image can be formed on the end surface of the lens, and furthermore, a lens with a short focal length can be provided.
(4) The optical axis of the lens coincides with the geometric center so that the lens can be easily adjusted.
Methods for imparting the refractive index distribution to a glass rod include ion implantation, molecular stuffing, and ion exchange method. In the case of rod lens, the ion exchange method is used so that the distribution becomes smooth and symmetrical.
Referring to FIG. 17, the ion exchange method employs a kiln 112 containing a fused salt 113 of high temperature. A glass rod 116 is immersed in the salt 113 so that an alkali ion A in the glass rod and an alkali ion B in the salt 113 are exchanged with each other. As a result, there is formed in the glass rod 116 an ion concentration distribution which is in proportion to the refractive index distribution described above.
However, the rod lens thus formed has the following problems.
(1) In order to manufacture the rod lens, there is a need to provide a device for the ion conversion treatment so that the manufacturing cost increases.
(2) The conjugation length TC, which is the distance between the original object and the image formed, can only be selected from the lineup of the rod lens products. Thus the distance TC cannot be shortened for manufacturing a thin contact-type sensor.
In order to solve the problem, there has been proposed a lens where a known liquid crystal lens shown in FIGS. 18a and 18b is used instead of the rod lens array. The construction and the features of the liquid crystal lens are described in a known publication OplusE., October, 1998, Vol. 20, No. 10, Kabushiki Kaisha Shingijutsu Communication, featuring liquid crystal optical elements and their applications: liquid crystal microlens.
In order to form an optical element which serves as a lens with a liquid crystal, a liquid crystal layer, which becomes a medium, may be shaped into lens as in glass lenses. Alternatively, the optical element maybe constructed so that a spatial refractive index may be imparted. In a nematic liquid crystal cell, liquid crystal molecules are aligned in the direction of an electric field. Thus, due to the distribution effect of the liquid crystal molecules in the electric field which is symmetric with respect to the axis and inhomogeneous, a lens having a spatial refractive index distribution can be provided. When such a liquid crystal lens is employed, a microlens array where a plurality of miniaturized lens are arranged in two dimensions in a flat plate is easily provided.
Referring to FIGS. 18a and 18b, the nematic liquid crystal cell 121 comprises a lower transparent glass substrate 123, an upper transparent glass substrate 122, a pattern electrode 124a on the lower transparent glass substrate 123, a pattern electrode 124c formed on the underside of the upper transparent glass substrate 122, a transparent alignment layer 125a on the electrode 124a, a transparent alignment layer 125b on the electrode 124c and an enclosing member 127 provided between alignment layers 125a and 125b. The pattern electrode 124a is formed by a conductive electrode film and has a plurality of circular holes 124b, and the pattern electrode 124c is also formed by a conductive electrode film and has a plurality of circular holes 124d. Each of the circular holes 124d is concentrically formed with an opposite hole 124b. A liquid crystal material 128 is injected into a space defined by the enclosing member 127 and the alignment layers 125a and 125b. The alignment layers 125a and 125b are rubbed so that the alignment of each layer is antiparallel and homogenous to one another. The pattern electrodes 124a and 124c are so disposed that the holes 124b and the holes 124d coincide.
When the liquid crystal cell 121 is applied with a voltage higher than a threshold, electric potentials are distributed as shown by contour lines in FIG. 19a. As shown in the figure, the electric field intensity has such a spatial distribution as to be increased as the distance from a center of the hole 124b (124d) of the pattern electrode 124a (124c) increases in the radial direction, that is, a distribution is symmetrical about the axis of the cell.
In FIG. 19b, a section of the liquid crystal material 28 is divided into a plurality of regions by the contour lines and the vertical division lines, and a typical director is shown for each region.
Namely, when a voltage larger than a threshold voltage is applied, liquid crystal molecules are aligned in a direction balanced by the resilience of the liquid crystal determined by the alignment layer and aligning force caused by the electric field. More particularly, as shown in FIG. 19b, the liquid crystal molecules are inclined at the maximum angle with respect to the horizontal direction of the substrate at a portion adjacent the periphery of the hole, and the inclination becomes smaller towards the center portion of the pattern. In other words, since the liquid crystal molecules are aligned along the electric field distribution which is symmetrical with respect to the axis, the effective refractive index is so distributed as to be decreased adjacent the periphery of the hole 124b of the electrode and to be increased at the center of the hole. Thus, although the liquid crystal lens has flat end surfaces, the lens has the characteristic of a convex lens.
When the liquid crystal microlens is applied to such a sensor as the sensor 110 in FIG. 14 instead of the rod lens, an image of the text can be formed at the receiving portion of the sensor so as to be scanned. The liquid crystal microlens is thinner than the rod lens so that sensor having a smaller thickness can be manufactured. Moreover, the liquid microlens can be produced by an easier method than the ion exchange method of the rod lens. In addition, by controlling the voltage applied to the electrodes, the refractive index of the liquid crystal can be controlled, thereby enabling to set a desired resolving power, depth of focus, brightness and focal length as required. Hence a handy lens can be provided.
However, when the liquid crystal microlens is adapted for a contact-type sensor for reading an image on a text, there occur the following problems.
(1) The electrode of the liquid crystal lens must be applied with voltage during the whole time the text is being read, thereby increasing the power consumption. Hence in a hand scanner for a facsimile machine driven by a battery, the life of the battery is decreased.
(2) Depending on whether the voltage applied to the liquid crystal microlens is alternating current or direct current, and on the level of the voltage, a liquid crystal driving IC must be provided in the power source of the sensor so that the manufacturing cost is increased.
(3) Since it takes some time for the molecules of the liquid crystal to align in the desired direction after the voltage is applied, the response time dependent on the response speed becomes necessary. Hence a waiting time is required for the scanner to start scanning so that the operability of the sensor is deteriorated.
An object of the present invention is to provide a liquid crystal microlens where the above described problems are resolved.
According to the present invention, there is provided a liquid crystal lens comprising a pair of transparent upper and lower glass substrates which are disposed apart from each other so as to form a space there-between, a pair of electrodes provided on the underside of the upper substrate and on the upper surface of the lower substrate, an alignment layer formed on each of the electrodes, at least one electrode having at least one hole, an ultraviolet curable liquid crystal material charged in the space, the liquid crystal material having a lens construction formed by an electric current and hardened by irradiation of ultraviolet rays.
Furthermore, there is provided a liquid crystal lens comprising a pair of transparent upper and lower glass substrates which are disposed apart from each other so as to form a space there-between, a pair of electrodes provided on the underside of the upper substrate and on the upper surface of the lower substrate, an alignment layer formed on each of the electrodes, each of the electrodes having a plurality of circular holes, each of the holes of one of the electrodes being opposed to the hole of the other electrode, an ultraviolet curable liquid crystal material charged in the space, the liquid crystal material having a lens construction formed by an electric current and hardened by irradiation of ultraviolet rays.
These and other objects and features of the present invention will become more apparent from the following detailed description with reference to the accompanying drawings.